In many applications, such as synchronous digital communications, metrology, etc. there is a need to transmit a highly stable clock signal from one point to another of the communication network or of the measurement system, e.g. from the generator of a main system clock signal to a utilization point. This signal can be transferred in many ways, e.g. via radio, by means of microwave links or through optical fibers. Clearly the latter way is adopted in digital communication systems which use optical fibers as physical carriers, e.g. the SDH (Synchronous Digital Hierarchy) networks. The clock signal can be transmitted explicitly, or it may be implicit in the data stream of a digital transmission and represent the timing of that stream.
For the sake of simplicity, the following description shall, in both cases, refer to "clock signals" or "clock information".
In the aforesaid applications, clock signals must be highly stable not only in frequency but also in phase and, considering by way of non limiting example digital communication systems, international standards set strict limits to the phase fluctuations of these signals. If clock signals are transmitted over an optical fiber, the phase of the signal received by the user is influenced by the variations in the length of the optical path and it depends essentially on temperature variations along the fiber. As is well known, this phase is given by .PHI.=2.pi.f.tau., where f is the frequency of the clock signal to be transferred and .tau. is the propagation time along the fiber, in turn given by .tau.=n.sub.g L/c, where n.sub.g is the group refractive index (which depends on wavelength and temperature) and L is the length of the fiber, which also depends on temperature. In particular, the dependence of n.sub.g on temperature and wavelength and the dependence of L on temperature can be expressed by relations: ##EQU1## where the terms with index "0" indicate the value of the respective parameter at given reference wavelength and temperature .lambda..sub.0, T.sub.0.
Since in conventional optical fiber transmission systems wavelength is usually fixed, it can be clearly seen that the sole parameter influencing the phase is temperature T.